In current system designs for mobile or wireless communication, in particular in the context of the 3rd Generation Partnership Project (3GPP), there is a clear tendency to optimize the standardized system primarily for maximum spectral efficiency and maximized data throughput. Additionally, latency is a key issue as well, which is also under consideration when specifying systems and their operation.
One-way latency in a communications system is defined as the delay from when a data packet is available for transmission in one end until it is available for further processing in the receiving end, while two-way or round-trip latency is the sum of the latencies in both directions, e.g. the time it takes for a response on a packet to become available at the transmitter of that packet.
Latency will limit the achievable data rate, and is inherent in all protocols, as e.g. TCP/IP, for basically all applications. As an example, whenever a handshaking between peers on both sides of a communication line is needed e.g. in a request-response way, one peer needs to wait until information is sent to the other peer and subsequently the answer is received. This waiting time, i.e. the latency, limits the achievable throughput in practice, even if an unlimited amount of data could be sent per time interval.
Latency is in particular an issue for multi-hop or relaying systems where data have to transverse several hops where each link between nodes will have its own individual Hybrid automatic repeat request (HARQ) loop, but latency could also be an issue for conventional single hop systems.
There are several ways to increase throughput, partly by increasing the spectral efficiency but mainly by exploiting more resources, e.g. using more base stations, more spectrum and more antennas per base station and user equipment (UE).
It is much harder to improve latency: the main tools are to reduce the time span of a physical layer packet (smaller Transmission Time Interval TTI, currently 1 ms in LTE after 2 ms in HSDPA and 10 ms in UMTS), decrease cell sizes in order to reduce the travel time of the signals (speed of light), or more generally put peer nodes as close as possible to the UE (e.g. from RNC in UMTS to the eNB in LTE) and to do the processing in the nodes quicker. The first cost spectral efficiency due to increased control overhead, while the second requires more base stations and more functionality to be implemented in them and the latter costs more powerful processing elements.
It is therefore a problem to reduce the latency in current communication systems, and thus there is still a need for an improved method for processing data in a radio communications network.